The present invention is generally directed to a system for focusing laser light on a selected layer in a single-layered or multilayer optical disk information storage system. More particularly, the present invention is directed to a system for providing an electrical signal which is proportional to an amount by which a laser light source is to be moved relative to a track on a rotating optical disk in order to provide enhanced tracking. Even more particularly, the present invention is directed to a system in which a light source is electromechanically dithered to produce an electrical signal which is distinctive in terms of its ability to provide an indication of the position of a focused spot relative to a track which is being read or written.
Use of optical disks for information storage has become quite popular. These disks are capable of storing digital data and are generally referred to as CD ROMS. These devices have been shown to be capable of storing many hundreds of megabytes of information.
Even more recently, it has been shown that it is possible to write and read information to multiple layers using optical disk storage technology. These new optical disks can store distinct information on each additional layer. This multilayer approach to optical storage significantly increases the storage capacity of each disk. However, new high-density drives are required to utilize these disks.
These new drives contain a laser and a lens mounted on a servo motor-controlled mechanism. By moving the lens toward and away from the optical disk, laser light is focused on different layers in the disk. However, the more layers that are employed in the disk for data storage, the more difficult it is to provide proper laser focusing. More particularly, as a result of the focusing difficulty, these disks typically require "buffers" of 100 microns or more in thickness between each of the data layers.
Accordingly, there is a need to be able to accurately focus laser light on to the optical disk and concomitantly to be able to read reflected light from that particular layer. It is, therefore, seen that it would be highly desirable to be able to employ an alignment technique using the servo motor and feedback loop which are already present in such drives. Furthermore, it is generally desirable to be able to improve tracking even for single layer optical disks in order to make them more tolerant to shock and vibration for use in portable or mobile applications. It is also very desirable to be able to provide a solution to these problems via a simple modification of disk drives which are already in use so as to improve their tracking accuracy.